Oceans and seas are
the bodies of salt water that cover about 71 percent of the Earth's surface
and are referred to in total as the world ocean. Several centuries ago
the "seven seas" were considered the navigable oceans, namely the Atlantic,
Pacific, Indian, and Arctic oceans, the Mediterranean and Caribbean seas,
and the Gulf of Mexico. At present, however, oceanographers consider all
other oceans and seas as belonging to the Atlantic, Pacific, or Indian
oceans. The Arctic Ocean, the Mediterranean and Caribbean seas, and the
Gulf of Mexico are considered marginal seas of the Atlantic Ocean. These,
in turn, have their own marginal bays and seas. Narrow, shallow straits
separate the marginal seas from the Atlantic: the Straits of Florida (Gulf
of Mexico), the Strait of Gibraltar (Mediterranean), and many gaps between
the islands of the Greater and Lesser Antilles for the Caribbean Sea.

Many other large bodies
of water have been designated as seas, but all are marginal to the great
oceans. The largest of these are the Bering Sea, the Coral Sea, the East
China Sea and South China Sea, the Sea of Okhotsk, the Sea of Japan, the
Yellow Sea, and the Philippine Sea, bordering the Pacific; the Arabian
Sea, the Red Sea, and the Bay of Bengal, bordering the Indian Ocean; the
Scotia Sea, the North Sea, the Labrador Sea, the Weddell Sea, the Norwegian
Sea, and the Greenland Sea, bordering the Atlantic Ocean. Marginal seas
differ from the major oceans primarily in size, but also in depth and
bottom topography.

The boundaries between
the oceans are based on geographic criteria and have little to do with
physical water-mass boundaries. The Atlantic is separated from the Indian
Ocean by the 20° E meridian, and from the Pacific Ocean (in the south)
by a line extending from Cape Horn at the tip of South America to the
South Shetland Islands off Antarctica's tip and (in the north) by the
narrowest part of the Bering Strait. The dividing line between the Pacific
and Indian oceans extends along an arc through the Malay Peninsula, Sumatra,
Java, and Timor to Cape Londonderry in Australia, to Tasmania, and then
along the 147° E meridian to Antarctica.

Reference is often
made to the Antarctic, or Southern, Ocean, which encircles the Antarctic
continent and consists of the southernmost sectors of the three principal
oceans. In spite of the lack of definitive geographic boundaries, the
meteorological and oceanographic conditions in the high southern latitudes
combine to produce a well-defined circumpolar current called the West
Wind Drift. This current distinguishes the Antarctic Ocean as a physical
entity, but the ocean's geographic borders are less easily defined.

Oceanic regions constitute
a much larger percentage of the Earth's surface in the Southern Hemisphere
(81%) than in the Northern Hemisphere (61%). This factor is reflected
by major differences in oceanic circulation and weather patterns between
the two hemispheres.

Origin
An explanation of the origin of the world's oceans must account for both
the great ocean basins as well as the source of the water filling them.
Perhaps surprisingly, neither the basins nor the volume of water in them
has remained constant over the history of the Earth.

The
Water. As the cosmic dust collapsed billions of years ago to form
the planet Earth, water was probably locked into rock-forming compounds.
These compounds (hydrated silicates) would have slowly released the trapped
water during the first billion years or so of Earth history and formed
the primordial ocean. The duration and time of initiation of this process
are not exactly known, because rocks containing the record of that time
span have been destroyed in the succeeding 3 billion years. Water would
not have been released at the earliest stage of the Earth's development,
however, because a molecule of water is lighter than a molecule of any
of the lighter elements, or "volatiles," such as neon, that would have
escaped into space during the intense heat that accompanied the formation
of the planet. On the other hand, the origin of the primordial ocean must
have occurred during the first billion years, because some of the earliest
rocks found on Earth show evidence of deposition in a large body of water.

The time required
to accumulate the volume of water in the present oceans is unknown. The
water was released during the cooling of the Earth and attained, early
after its initiation, a volume not drastically different from that of
the modern oceans. That the volume of water in the oceans has not changed
drastically during the last few hundred million years is inferred from
evidence indicating that the interiors of the continental land masses
have never been inundated by deep oceans. Any incursions of seawater upon
the continents that did occur were in response to tectonic changes (that
is, changes due to deformations of the Earth's crust) rather than changes
in the volume of water in the oceans.

The most recent changes
in ocean volume have accompanied the ice ages during the last two to three
million years. The northern polar icecap has expanded and contracted with
great regularity, and the volume of the oceans has fluctuated correspondingly
as water is alternately locked into or released from the ice cap. These
fluctuations, however, have accounted for changes in sea level of no more
than 200 m (660 ft).

The
Basins. The events of formation of the initial ocean basins that held
the primordial ocean are unknown, because they have been destroyed by
subsequent geologic events. The history of the present ocean basins, however,
is well known. All of the continental land masses as they are now known
began to break up about 200 million years ago from two great supercontinents
called Gondwanaland and Pangea. As the continents drifted to their present
positions, the great ocean basins were left in their wake. This concept
has been refined within the framework of the theories of continental drift
and plate tectonics.

Each ocean basin has
evolved in a slightly separate manner, but the overall history of each
basin has a similar series of events. The origin of the Atlantic Ocean
basin is best known and serves as a good example of the processes involved.
About 150 million years ago forces at work beneath the crust of the Earth
split the supercontinents into large fragments. One such fragment contained
North America joined to Eurasia, and another contained South America joined
to Africa. By 135 million years ago North America had separated from Eurasia,
leaving a small, restricted ocean basin with no open ocean circulation.
At the same time, the early South Atlantic basin was formed by the separation
of South America and Africa. The two parts of the Atlantic were not connected
with a pathway for deep circulation until about 65 million years ago,
after the South Atlantic had widened to approximately 3,000 km (1,800
mi) and the North Atlantic had widened as a result of a separation between
Greenland and Europe. The process continues today as new (basaltic) ocean
crust is added at the Mid-Atlantic Ridge, widening the Atlantic Ocean.

The same process has
formed the Pacific Ocean basin, but the relatively simple pattern outlined
for the Atlantic Ocean has been complicated by other factors in the Pacific.
The Pacific Ocean basin is surrounded by the great marginal trench systems,
which are accompanied by volcanic island arcs and violent earthquake activity.
These trenches mark the location where great slabs of oceanic crust are
being reabsorbed back into the Earth by a process called subduction, which
generates the seismic activity along the boundaries of the Pacific Ocean
basin. This process is active along the Aleutian Trench to the north,
the Kuril-Japan-Marianas Trench System and the Mindanao Trench to the
west, the New Hebrides, Tonga, and Kermadec trenches to the south, and
the Middle American and Peru-Chile trenches to the east. New oceanic crust
is being produced in the Pacific Ocean basin along the East Pacific Rise
in the southern and eastern parts of the basin.

The history of the
Indian Ocean basin has not been completely determined. India and Australia
separated from Antarctica about 80 million years ago. India slid past
Australia about 45 million years ago and rammed into Asia, leaving behind
a deep ocean basin that widened in an east-west direction beginning about
35 million years ago. The pattern of seafloor formation, however, in this
relatively small ocean basin has not been completely determined, due to
the complexities found there.

Physiography
of the Seafloor
The seafloor is shaped into a host of volcanic features that grade from
long submarine mountain chains, which are larger than their continental
equivalents, to deep trenches that are thousands of times larger than
those found on land. The shape of all these physiographic features is
related to the origin of the slice of ocean floor on which they are found.
The shape of the seafloor, in turn, affects the origin and distribution
of some of the great oceanic circulation systems. These systems play a
role in the distribution of the oceanic nutrients that control biogenic
productivity in the oceans.

Ridges,
Plains, and Trenches. Perhaps the most striking feature on the seafloor
is the mid-oceanic ridge system that, through branches, extends across
all the major ocean basins. Typically, the central axis of this system
is marked by a steep- walled valley, usually about 40 km (25 mi) wide
and 2 km (1 mi) deep. Small segments of the ridge extend above sea level
to form islands (for example, Iceland, the Azores, and the Galapagos Islands),
but most of the ridge crest is at a depth of approximately 2.5 km (1.5
mi).

The seafloor continues
to deepen away from the crests of the mid-oceanic ridge system out to
the extensive, flat abyssal plains, which constitute the largest segment
of the seafloor. The depths of the individual plains are roughly uniform
the deepest of them occur in the Pacific Ocean (6,000 m/20,000 ft), and
the shallowest occur in the Atlantic Ocean (5,000 m/ 16,000 ft).

In selected areas,
usually at the margins of the ocean basins, the abyssal plains descend
into steep oceanic trenches, where the greatest depths in the oceans are
found. Examples of these trenches include the Peru-Chile Trench (Pacific),
8,055 m (26,428 ft); the Puerto Rico Trench (Atlantic), 9,200 m (30,200
ft); the Tonga Trench (Pacific), 10,880 m (35,700 ft); and the deepest
hole in the ocean, the Marianas Trench (Pacific), 11,022 m (36,163 ft).
Associated with most of these major trench systems are volcanic island
arcs found on the landward side of the trenches.

Fracture
Zones and Seamounts. Superimposed on the features of the seafloor
previously mentioned are many other smaller-scale, but very important,
physiographic features. Segments of the mid-oceanic ridges are commonly
offset laterally by parallel, linear fracture zones thousands of kilometers
long. These oceanic fracture zones are characterized by ridges and valleys
separated by steep rock cliffs that are hundreds to several thousands
of meters high. These fracture zones can be traced out into the abyssal
plains, where the traces of the cliffs are lost beneath the sediment.

The abyssal plains
are also dotted with numerous isolated mountains called seamounts that
extend in some cases above sea level to become islands. Characteristically,
these seamounts belong to large groups of such features, which may be
randomly dispersed over a large area or arranged in a line.

Seafloor
Spreading. The origins of many of these strikingly different physiographic
features are linked together by the concept of seafloor spreading. The
seafloor is produced by the cooling of upwelling molten material at spreading
centers characterized by the mid-oceanic ridges. The ridges are broken
and discontinuous due to fracture zones along which transform faulting
occurs. Newly formed material moves away from the ridge crest, and tens
of millions of years later it is subducted or downwarped in an oceanic
trench.

Continental
Shelves, Slopes, and Rises. The margins of all the continents extend
seaward as a broad, flat, shallow shelf. These continental shelves dip
gently seaward and are usually less than 200 m (660 ft) below sea level.
The continental shelves comprise only a small portion (7.6%) of the seafloor,
but their importance is far greater, because the shallow depths make it
possible to exploit their natural resources.

The shelves abruptly
end at the shelf breaks, where the seafloors rapidly descend along the
continental slopes to the abyssal depths.
At the bases of the continental slopes, which cover about 15.3% of the
oceanic area, are the continental rises, which represent a series of sediment
slumps from the slopes above that have spilled down onto the deep seafloor
or ocean basins. The basins cover about 75.9% of the oceanic area. Only
1.2% of the ocean is greater than 6,000 m (19,686 ft) in depth.

Importance
of the Oceans and Seas
Oceans and seas are now understood to be integral parts of the entire
geologic process of continental weathering, runoff, and deposition, followed
by either uplift and subaerial exposure or subduction into the depths
of the Earth during the process of plate tectonics and seafloor spreading.

The oceans and seas,
also known as the hydrosphere, are responsible for the regulation of many
major processes that occur on the surface of the Earth. Much of the precipitation
that falls upon land areas is derived from oceanic evaporation. The hydrosphere
acts as a tremendous heat reservoir, exerting a dominant effect on temperature
extremes over large land areas. The movement of ocean currents also creates
moderating effects in some areas at latitudes where weather extremes might
otherwise make life unpleasant. In addition, the oceans act as reservoirs
for numerous other substances that provide a buffering effect on the levels
of various gases in the atmosphere and, in some cases, a dilution of otherwise
toxic materials that humans have introduced into salt and fresh waters.

The oceans and seas
represent a place of recreation, a means of transportation, and a storehouse
of food, mineral resources, and energy sources. Their potential as a source
of immeasurable resources is just beginning to be realized.

Transportation.
The ocean provides tremendous potential as a means of transportation.
The major portion of goods distributed worldwide are shipped by water
the least expensive method of transport, and one that provides a livelihood
for a significant percentage of the world's population.

Food.
As a biological entity, the oceans represent a highly productive environment.
The basic biological habitats of the ocean can be divided into pelagic
(water region) and benthic (seafloor region) environments. The pelagic
zone can be divided into the neritic zone (down to 200 m/660 ft and over
the continental shelf) and the oceanic zone (below 200 m). The neritic
and the upper oceanic ( epipelagic zone) regions correspond to the photic
zone, in which photosynthesis is possible. The benthic habitat is also divided into the intertidal zone (between high and low tide),
the sublittoral zone (down to 200 m; see littoral zone); the bathyal zone
(200 to 4,000 m), the abyssal zone (4,000 to 5,000 m/13,000 to 16,000
ft), and the hadal zone (deeper than 5,000 m). The photic zone is the
most highly productive area, containing numerous benthic, planktonic (free-floating),
and nektonic (free-swimming) marine creatures. Below the photic zone the
biomass decreases considerably, with the only food source for marine life
being the constant rain of organic matter from above. The lack of light
and food has produced numerous adaptations in deep-sea life, resulting
in some very strange forms at these depths.

Throughout recorded
history people have used the ocean as a source of food. But even at present
rates of removal, the food-resource potential of the oceans has barely
been touched. At present, most countries remove only certain choice species
of fish from the ocean. This practice has led to the depletion of fish
populations of these few species, whereas at the same time other species
have remained almost untouched. The depletion of fish populations in once-choice
fishing grounds has caused considerable international dispute between
the governments and fishing fleets of many countries ultimately leading
to expansion of offshore fishing limits in many parts of the world and
has also forced the development of techniques for fish farming and the
cultivation and seeding of many bottom areas in order to enhance shellfish
production.

A change in attitudes
about the consumption of various species of fish may also help alleviate
fishing pressures on certain species and encourage a more balanced exploitation
of fisheries resources. The infancy of research in fisheries biology is
such that the behavior, food requirements, and breeding habits of most
fish are not well known. Investigation may lead to a situation where human
beings can depend more heavily on the ocean as a food source without creating
a drastic impact on the overall ecology of the ocean.

Water.
The ocean is a source of fresh water in many highly arid, nearshore areas
where the cost of transporting water from regions where naturally occurring
fresh water is abundant is greater than the cost of desalination. Most
current desalination methods resemble a distilling process. Other methods
that are available but not in widespread use are freezing, reverse osmosis,
and ionic processes. Current costs of desalinization are not so high as
to be prohibitive to domestic and some industrial users; but whether new
developments will bring costs down to a level where agricultural and other
industrial users will find the use of desalinized water practical remains
uncertain.

Energy.
Theoretically, the ocean represents a tremendous source of energy. Current
use of the oceans by the energy industry is restricted for the most part,
however, to the use of seawater as a coolant in nearshore nuclear power
plants. This particular use has produced considerable response from groups
concerned with the environmental impact of the discharge of heated water
from these reactors.

Some energy can be
extracted from the ocean by making use of the change in sea level caused
by tidal cycles. The use of this method, however, is currently limited.
Numerous other methods are mostly in experimental stages at present. Several
of these methods make use of the temperature differential that exists
across the thermocline. Other methods would make use of wave and current
energy. Most of these designs suffer from one or more disadvantages, such
as high generation costs, great distances between suitable generating
sites and the power market, or a number of engineering problems.

The Sun replenishes
the oceanic energy reserves at a much greater rate than human beings could
ever remove energy from the oceans. The problems of economically removing
this energy, most of which is diffused over the entire ocean, are, however,
at present so insurmountable as to make widespread dependence on this
energy resource impractical.

Oil.
Since the 1960s the production of oil from wells on the continental shelves
has increased drastically, to the point where it represents a substantial
percentage of the world's production. More recent evidence indicates that
oil deposits also reside on the continental slope. Current drilling and
maintenance technology, however, has not yet advanced to a state where
tapping of these additional reserves is practical in a routine manner.
Current estimates of offshore reserves far surpass presently known onshore
reserves. A need for additional energy supplies will no doubt stimulate
the technological advancements necessary to take advantage of these reserves
and to minimize damage to the environment.

Minerals.
Economically important minerals are constantly introduced into the ocean
from a variety of sources, and most of the material accumulates on the
ocean bottom. Rivers dump vast quantities of particulate mineral materials
into the oceans each year. Volcanic eruptions and hydrothermal solutions
introduce many metals into solution and in solid form.

Heavy minerals have
a tendency to accumulate in superficial placer deposits, which can be
located using a variety of geophysical techniques and then can be mined
by dredging. Phosphate rocks are also mined from nearshore areas. Salt
domes, which are subbottom bedrock deposits, are mined by pumping hot
water down to melt the sulfur and to force the molten material back up
the drill string.

The deep sea, as well
as nearshore environments, stores vast amounts of economically important
oceanic mineral resources. Large areas of the ocean bottom, mostly in
temperate and tropical regions, are covered with calcareous deep-sea oozes,
which are the product of deposition of calcium carbonate skeletons accreted
by planktonic and benthonic microorganisms. This material could be useful
in the manufacture of various building supplies, most notably cement and
concrete. In higher latitudes the ocean-bottom sediments are dominated
by siliceous oozes. These oozes, the product of silica minerals secreting
microorganisms, serve as an efficient filtering and insulating material.

Ocean
Pollution
Oceans clearly play an essential role in life on Earth, yet because of
their vastness humans have tended to use their waters as dumping grounds
for many waste materials. This practice has increased as land areas for
such wastes diminish. Oceans also receive all of the pollutants that are
fed to them by the rivers of the world. Even when ships are not actively
engaged in dumping wastes, they are themselves sources of pollution most
notably, the giant tankers that have caused numerous massive oil spills.

As a result, by the
late 20th century ocean studies indicate that what had once been thought
impossible is becoming a reality. The oceans as a whole are showing signs
of environmental pollution. Even the surface waters of the oceans are
increasingly plagued by obvious litter. Some of this litter washes ashore
to render beaches unsightly, while other such debris entangles and kills
many sea birds and mammals every year.

More insidious than
these litter problems are the effects of toxic contaminants from wastes
that are dumped in the ocean. These chemicals can upset delicate marine
ecosystems as they are absorbed by organisms all along the food chain.
Even the paints that are being used on many ships can be hazardous.

The need to address
the matter of ocean pollution has been recognized at national and international
levels. The U.S. Congress, for example, passed an act in 1988 that would
prohibit ocean dumping by 1991, and in that same year 65 nations agreed
to cease burning toxic wastes at sea by 1994 should acceptable alternative
practices be found. In the late 1990s the latter action remains under
debate, however, as several nations continue the practice of ocean burning
of toxic wastes. The U.S. act may prove as unenforceable as a prior one
in 1977 that attempted the same prohibition. Worldwide, the problem of
ocean pollution remains.

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